This invention relates to flame retardant blends of polysulfone, with polyalkylene phthalate The present blends are flame retardant even at thin part thickness.
Aromatic polymers such as polyethylene terephthalate, polyarylsulfones, and poly(aryl ether) sulfones, in admixture with themselves, or when blended, do not exhibit adequate flame retardancy. As a result, various flame retardant additives have been disclosed which render such compositions flame retardant in varying degrees of effectiveness.
There are many known flame retardant additives which are employed by mixing them with other materials such as polymers to render materials flame retardant. Such flame retardant additives have been known to be employed in amounts of at least 1 weight percent in order to provide flame retardant characteristics to those products which are combustible.
The use of compounds containing phosphorus and/or nitrogen as flame retardant additives for some polymers has been recognized, as has been the use of various halogenated materials, such as chlorinated paraffin wax, and antimony compounds such as antimony oxide, and mixtures thereof. One disadvantage, however, in regard to the known materials which have been used as flame retardants has been the fact that generally a large amount of the additive must be incorporated into the polymer to make it sufficiently flame retardant. In addition, flame retardant additives frequently do not stand up to processing conditions and will in some cases separate out from the resin after incorporation. The search for fire-retarding adjuvants is important because it is essential that many resin compositions have relatively high resistance to burning if they are to be commercially utilizable. Further, the resins must be capable of enduring heat without deterioration and be able to resist fire and flame. Especially, materials used in conjunction with electricity must be able to resist ignition or deterioration by heat and sparks. As the requisite degree of flame retardance is achieved, it is essential that the other desirable qualities of the resinous material be preserved or enhanced. In addition to the preservation of the mechanical properties of the polymer, it is desirable that the color should not be adversely affected.
Flame retarding additives such a triphenyl phosphate or aluminum trihydrate which generally possess low flammability have been mixed with engineering thermoplastics to reduce flammability of the thermoplastics. However, a blend of such a low flammability additive with high performance engineering thermoplastics often does not yield a useable flame resistant composition. For example, the low flammability additive may not be compatible, with the engineering thermoplastic, at additive concentrations achieving significant flame retardance, resulting in lower levels of flame resistance, or the additive may not be stable at the processing temperatures of the engineering thermoplastic. Furthermore, low flammability additives which are compatible with a particular engineering thermoplastic often cannot effectively lower the flammability of the thermoplastic. If the effect on flammability is merely a reduction due to dilution, then amounts of the low-flammability additive necessary to achieve a desired reduction in flammability can adversely affect the physical properties or processibility of the engineering thermoplastic.
Perfluorocarbon polymers have been used in the past to inhibit drip in thermoplastics and improve heat and smoke release in polysulfone. EPO307,670 to Rock discloses that polysulfone with 10 percent perfluorocarbon added has improved heat and smoke release characteristics. It does not show blends of polyester with polysulfone or the advantage of adding borate salts to the combination. Further, it does not disclose UL-94 test data. EPO400,935 discloses flame retardant glass fiber reinforced polyesters including polyalkylene terephthalate and another polymer. The synergistic flame retardant use of PTFE and borate salts are not taught or suggested. Thermoplastic blends of aromatic polysulfones and thermoplastic polyesters are known from Nield, U.S. Pat. No. 3,742,087. Nield states that the blends may be mixed with particles of other polymeric materials as fillers having special properties, e.g., elastomeric materials and polytetrafluoroethylene (PTFE). There is no disclosure of the difficulty of achieving flame retardancy (UL-94 at V-1 to V-0) in thin part thickness (e.g. down to 60 mils) plastics nor the use of polytetrafluoroethylene in conjunction with zinc borate in order to achieve flame retardancy of thin films of the present blends. Blends of poly(aryl ether), polyester and a compatibilizing amount of polycarbonate are disclosed in Robeson, U.S. Pat. No. 4,369,136. Robeson discloses that flame retardants may be used but does not delineate the particular flame retardants of the present invention or the advantages thereof in regard to thin films. In U.S. Ser. No. 753,188 an application by the present inventor, compatibilized and miscible blends of poly(aryl ether) and/or polycarbonate, polyarylate, and polyester are disclosed. In EPO364,729, a polycarbonate composition with PTFE, boric oxide and a graft copolymer of methyl methacrylate, butadiene and styrene and/or styrene-maleic anhydride copolymer were shown to have a UL-94 of V-0, however polycarbonate with PTFE and boric oxide, without styrene, only achieved a UL-94 of V-1 at 1.6 mm.
In Saito, U.S. Pat. No. 4,820,761, polysulfone with PTFE is disclosed as an aid for low mold shrinkage and good mold releasability. There is no mention of the present blends or the use of borate salts.
Zinc borate has been used in various thermoplastic compositions. Cella, et al., U.S. Pat. No. 4,833,190 discloses use of zinc borate as a smoke suppressant and flame retardant in silicone containing compositions. Anderson, U.S. Pat. No. 4,049,619 discloses a thermoplastic composition of a polysulfone, a flame retarding bisphenoxy compound and an enhancing agent, which is disclosed as including zinc borate.
Anderson, U.S. Pat. Nos. 4,041,013 and 4,049,619 disclose plastic compositions containing polysulfone and bisphenoxy compounds along with certain enhancing agents including zinc borate, the preferred enhancing agent being antimony trioxide. There is no teaching of a preference for zinc borate or its synergistic use with PTFE as flame retardants in the present thin film blends. In U.S. Pat. No. 4,981,895 to Buchert, a molding composition consisting of polyether sulfone or polyether ketone or mixtures thereof in conjunction with zinc borate were said to have improved heat release characteristics. No mention of the particular polysulfone of the present invention or its blend with polyester of PTFE were mentioned. This is significant since blending polyalkylene phthalate with polysulfone while having distinct advantages, significantly reduces flame retardancy of neat polysulfone.
Blends of polysulfone and other polymers such as polyalkylenephthalate combine the advantageous properties of impact resistance, hydrolytic stability, dimensional stability, and heat resistance of the polysulfone and yet can be tailored to lower the cost and not interfere with the advantageous properties. Blends of poly(aryl ether) and polycarbonate or polyester are known from U.S. Pat. Nos. 3,365,517, 3,742,087, 4,369,136, 4,371,672, and 4,746,710. It is shown in these patents that as a result of the blend the polymers are rendered more resistant to environmental stress crazing and cracking their heat distortion temperature is increased and the poly(aryl ether) is made more resistant to thermal stress and aging embrittlement. Furthermore, improved hydrolytic stability is disclosed. Thus, it can be seen that alloying of poly(aryl ether) sulfone with polyalkylenephthalate leads to materials with improved physical characteristics.
Unfortunately, by adding polyalkylenephthalate to polysulfone flame retardancy is significantly diminished. Thus, it has become particularly critical to improve the flame retardancy of the composition, first of all, in order to bring it up to par with the known flame retardant compositions utilizing polysulfone in the industry.